For example, as shown in Patent Document 1 (Japanese Patent Application Publication No. H03-212141), an axial air-gap electronic motor is a motor in which a rotor is arranged opposedly on one side surface or both side surfaces of a stator with a predetermined gap being provided therebetween, and is characterized in that the thickness thereof in the rotating shaft direction can be decreased, namely, the motor can be made flat as compared with a radial air-gap electronic motor such as an inner rotor type.
Usually, the rotor consists of a rotor back yoke formed by a disc-shaped body and a rotor magnet attached integrally to the surface of the rotor back yoke, which faces the stator, and the rotor magnet is attached integrally to the rotor back yoke with an adhesive or the like.
Also, a brushless motor including the axial air-gap electronic motor is provided with rotation position detecting means for detecting the rotation position (rotation phase) thereof. As one example, in Patent Document 2 (Japanese Patent Application Publication No. S63-92250), a position detecting magnet is provided on the outer peripheral surface of the rotor, and also a position detecting sensor is provided around the rotor.
Also, as another method, Patent Document 2 has disclosed a technique in which the position detecting sensor is installed on the back surface side of the rotor back yoke to carry out position detection (sensing). Further, as still another method, Patent Document 3 (Japanese Patent Application Publication No. S62-189960) has proposed a brushless motor in which the position detecting sensor is arranged in the coil of the stator.
However, the conventional axial air-gap electronic motor has problems as described below. For the axial air-gap electronic motor, the rotor is attracted to the stator side by the magnetic attraction force of the coil at the time of operation. Therefore, if the rotor magnet is fixed merely by bonding as described in Patent Document 1, the rotor magnet may peel off. The rate of occurrence of this peeling-off phenomenon increases as the motor torque becomes high.
Also, in the method described in Patent Document 2, the magnet and attachment member used exclusively for position detection are required, so that not only the manufacturing cost increases accordingly but also the weight of the rotor itself increases because of the additional installation of the detecting magnet, thereby increasing electric power consumption.
Also, in the case where the position detecting sensor is installed on the back surface side of the rotor, a communication hole is formed to obtain a magnetic gap in a part of the rotor back yoke. In the hole forming process, the position of the communication hole varies easily, and also it is difficult to perform highly accurate sensing because the distance between the magnet and the position detecting sensor becomes long.
Further, in the case where the position detecting sensor is installed on the stator side as shown in Patent Document 3, there also arises a problem in that the position detecting sensor is easily affected by a magnetic field generated by the stator, so that exact position sensing cannot be performed.